1. Field of the Invention
The present invention relates to a reusable semiconductor nonvolatile memory device, and more particularly to setting current values of a reference current.
2. Description of the Related Art
Semiconductor nonvolatile memory devices (hereinafter referred to as semiconductor memories) are widely used as memories for portable devices and the like, because nonvolatile memories do not need electric power to retain memorized information. In recent years, as memory cells have been miniaturized, optimum control of writing and reading conditions of the memory cells has become very important for improving reliability, productivity and the like of memories. Optimum control is realized by storing trimming information, which designates supply voltages and supply currents for incorporated circuits, on a memory chip in advance and referring to the trimming information during memory operations. As a method for storing trimming information, laser fuse trimming, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 11-17010, is commonly implemented. In this method, fuse elements are used for storing the information. Specifically, fuses are selectively cut in a wafer test process and, during operations of that memory, logical signals of 0 and 1, depending on whether or not each fuse is cut, are generated to serve as the information.
A technique for making this kind of nonvolatile memory reusable (which may hereinafter referred to as reuse) is disclosed in JP-A No. 2009-80872. In this technique, a nonvolatile memory that cannot be rewritten electronically is divided into a plural number of blocks, and the nonvolatile memory is reused by the blocks being expendably used.
The technique described in JP-A No. 2009-80872 expends the blocks. Therefore, the memory capacity is reduced with each reuse.
There is a reuse method in which data written to a nonvolatile memory is deleted. However, in the device mentioned above in which fuse elements are cut to store trimming information, conditions necessary for reading at a time of reuse may be shifted from the conditions specified by the trimming information that was stored in the fuse elements at the time of the wafer test, and the nonvolatile memory may become unreadable. This point is discussed below.
Heretofore, trimming information has been stored (for example, fuse elements are cut) at the time of a wafer test that is performed before shipping of the nonvolatile memory (see FIG. 3). Thus, a current value REF0 of a reference current (hereinafter referred to as the reference current) that is to be compared with cell currents from memory cells when data is being read from the nonvolatile memory is specified. The current value REF0 is determined by estimating a variation of retention (charge retention), from memory cell characteristics when the memory cells are first used (at a first time of use after shipping), and distributions of expected value 0 and expected value 1 after writing, taking account of a margin required for reading (see FIG. 12A).
When the memory is reused after the first use, the distributions of expected value 0 and expected value 1 after writing may be shifted from the time of first use, even if the cells are written under the same conditions as at the time of first use, because, for example, electrons and holes may not be properly neutralized or the like. Consequently, the margin required for reading may not be assured with a reference current of the current value REF0 that was specified by cutting the fuse elements. Therefore, reading reliability falls.
Now a principal reason that the margin required for reading may not be assured is described in detail. When a nonvolatile memory in which electrons have been injected and data has been written at the time of first use is to be reused, firstly, holes are injected and charge is neutralized to erase the written data. After the holes are injected, the nonvolatile memory may be heated (burning) in order to promote charge neutralization and improve retention. Charge neutralization is promoted if the memory is burned at a higher temperature, but after package assembly the burning may not be performed at a high temperature. Therefore, the burning is performed at a low temperature to neutralize the charges. A long duration of burning is required with a low temperature, and if the duration of burning is shortened, the electrons may not be properly erased. In these circumstances, the distributions of expected value 0 and expected value 1 when the memory is reused are shifted from the time of first use.
The variation of retention also changes rather than being constant. Thus, when memory cells are reused, the memory cell characteristics may not completely return to their original state and the situation described above may arise. With nonvolatile memories such as flash memories and the like, a process such as burning or the like is not carried out. Even so, memory cell characteristics may change with repeated writing and erasure and the situation described above may arise.
In nonvolatile memories, there are memory cells in which charge accumulation portions are provided at portions below gates as illustrated in FIG. 13A, and there are memory cells in which charge accumulation portions are provided at the sides of gates as illustrated in FIG. 13B and FIG. 13C. In a semiconductor nonvolatile memory of a side type in which the charge accumulation portion is provided at the side(s) of the gate, neutralizing charge is more difficult, and electrons are likely to remain even after erasure processing is carried out. Therefore, the situation described above is particularly likely to arise in side-type nonvolatile memories.
In the above descriptions, descriptions are given comparing a time of first use with a time of reuse, but this is not a limiting example. For example, an amount of shifting may be within a tolerance range and the margin required for reading may be assured up to a plural number of reuses from the first use, while thereafter the amount of shifting may exceed the tolerance range and the margin required for reading may not be assured.